Roller marks are parallel incuse lines that are formed on the metal surface of the coin during processing at the Mint. To understand how roller marks are made, we need to delve into the minting process a little bit.
Gigantic Metal Ingots - Before coins can be struck, the round metal discs must be prepared. These discs are called coin blanks, and they are stamped from large sheets of rolled out coin metal. The sheets of metal start as gigantic ingots of the coin alloy, made by pouring the molten alloy into huge molds. Once the alloy in the molds cools down enough to process, the mint workers remove the ingot from the mold and begin passing it back and forth through large metal rollers. The rollers are set a certain width apart, which is designed to flatten out the ingot of metal little by little, until it becomes the proper thickness for making the coin blanks.
Little Bits of Grit - Each time the metal ingot passes through the rollers, the ingot gets a little thinner, because the rollers are moved a little closer together. When the ingot has been rolled this way enough times, it becomes the proper thickness from which to punch out the coin blanks. In order to cause a thick ingot of metal to get thinner, however, the rollers must exert a great deal of force. If any tiny little piece of stray matter gets onto the ingot or the rollers, (such as little shavings of metal, or dirt or sand,) the incredible pressure from the force exerted by the rollers will cause the foreign matter to carve fine lines or grooves into the sheet of coin metal.
Long, Straight, and Parallel Lines - There are a few other events which can happen in the mint that will cause similar looking damage to the coin planchet, but roller lines can be distinguished from these other types of damage by the fact that roller lines are straight and parallel. The lines are long; in fact, they are longer than any single coin. By their nature, they should have affected the entire sheet of metal passing through the rollers, although some particles probably disintegrate eventually due to the great force. It is worth noting that damage from the "drawing press" is also included in the category of "roller marks" by most collectors.
Damage After Striking - The reason it is important to understand that roller marks are long, straight, parallel lines is so that you can differentiate them from other types of damage to the planchet, particularly damage that occurs after the coin leaves the Mint. One of the common post-minting conditions that is often confused with roller marks is cleaning marks. If an abrasive polish or rubbing cloth was used to clean the coin, there will often be marks left in the fields of the coin that appear similar to roller marks. However, polishing marks are usually not long and straight; they tend to be shorter in length, not perfectly straight, and sometimes lack parallelism.
Weight Adjustment Marks on Coins - Sometimes weight adjustment marks are confused with roller marks. Weight adjustment marks are file marks left by mint workers who used a file to remove excess metal from overweight planchets. This was an early practice in the history of minting United State coinage, and planchet weight adjustments of this type haven't been made on silver since about the 1840s. Also, adjustment marks lack the neat parallel nature of roller marks because the mint workers rarely took care to make neat marks. Adjustment marks are haphazard, multidirectional, and of varying depth.
Roller Marks go Underneath the Devices - Another diagnostic for confirming roller marks are that they will go underneath (and sometimes across) the devices on the coin and continue on the other side. The reason is that, as a feature of the planchet itself, when the coin is struck the lines are usually obliterated by the devices rising from the surface of the planchet during striking. The roller marks on the field area of the coin are often still present because the force of the strike wasn't enough to obliterate them. Large coins are more difficult to strike well, which is why roller marks are far more common on large coins such as Silver Dollars, than they are on smaller coins like Seated Dimes.
Roller Marks and Adjustment Marks - Comparison Diagnostics - I have prepared an image gallery of coins showing typical roller marks and adjustment marks, so that you can compare them. Note that adjustment marks are non-parallel straight lines, some of which are fairly deep, whereas roller lines are more shallow, often showing as numerous very fine lines, but which are always straight and parallel. (Shallow fine lines which are not straight and parallel can be signs of cleaning.)